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 Catalina/Capri 25/250 Sailor's Forums
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 Tohatsu high thrust propeller		  New Topic  Topic Locked
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JB
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USA
110 Posts
Initially Posted - 08/03/2018 :  08:12:06  Show Profile
There are two four blade props a 5 pitch and a 7 pitch with the same diameter of 8.7. Which appropriate for a C25?

I'd like to hit 5.5 knots at around 3/4 throttle. I do a lot of distance motoring and need to reach hull speed without over-revving the motor. I think I need someone's empirical results.
1988 C25 Wing Keel Std Rig Tohatsu 9.9 Tiller Steering and 2003 C250 Wing Keel Std Rig Inboard Diesel Wheel Steering

islander
Master Marine Consultant

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USA
3992 Posts
Response Posted - 08/03/2018 :  12:41:00  Show Profile
Tahatsu what? 8, 9.8, 9.9.....
Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound


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JB
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USA
110 Posts
Response Posted - 08/03/2018 :  13:37:37  Show Profile
Tohatsu 9.8 4 Cycle.
1988 C25 Wing Keel Std Rig Tohatsu 9.9 Tiller Steering and 2003 C250 Wing Keel Std Rig Inboard Diesel Wheel Steering
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islander
Master Marine Consultant

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USA
3992 Posts
Response Posted - 08/03/2018 :  13:44:17  Show Profile
I just bought a new 9.8 extra long(25") electric start this year and it has a 4 blade high thrust on it. 8.7" Dia. X 5.0" pitch. Pushes the boat along very well at the speed and revs you are looking for.
Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound


Edited by - islander on 08/03/2018 13:46:23
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bigelowp
Master Marine Consultant

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USA
1736 Posts
Response Posted - 08/03/2018 :  18:37:15  Show Profile
I have a 07 Tohatsu 9.8 and it came with a three blade prop which works well --so-- I doubt that the difference in pitch will make a significant difference.
Peter Bigelow
C-25 TR/FK #2092 Limerick
Rowayton, Ct
Port Captain: Rowayton/Norwalk/Darien CT
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Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts
Response Posted - 08/03/2018 :  19:16:54  Show Profile
5" is probably right for a displacement hull, and 7 for a planing hull that can exceed "theoretical hull speed". (Think of it as a tractor versus a pickup truck.) But I wouldn't fixate on running at hull-speed (6.3 kts) unless you don't care about MPG or noise. At that speed, the transom is dragging, which greatly increases the wake, which represents drag and wasted energy. The lower pitch will give you better thrust for maneuvering around docks, and will let the motor work less at cruising speeds below 6.3 kts. (Under load, a higher RPM at lower torque can be more efficient than the opposite.)

Either prop can probably get you to hull speed at max throttle, but 5.5 kts is a better target for most cruising (unless you're trying to outrun something). If you consistently need to do better than that, get a boat like my current one (below).
Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge
Edited by - Stinkpotter on 08/03/2018 19:33:51
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Voyager
Master Marine Consultant

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USA
5231 Posts
Response Posted - 08/04/2018 :  07:39:21  Show Profile
The only time I open up the engine full throttle is in a following sea in big chop. I prefer to ride the troughs without getting pooped. That’s usually only for short spurts at a tidal inlet. It kills fuel mileage though so I normally motor at around 4.5 to save fuel. If I had a long run out to my sailing area (I used to have a 30 minute run), I’d find another marina (I did).
My neighbor has the best of both. He has a MacGregor 26 sailboat sporting a 25HP Honda.
Bruce Ross
Passage ~ SR-FK ~ C25 #5032

Port Captain — Milford, CT
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islander
Master Marine Consultant

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USA
3992 Posts
Response Posted - 08/04/2018 :  08:01:46  Show Profile
When I was buying my motor I contacted Tohatsu about the props. A rep said that since the engine was a 25" they equip it with the 5" pitch assuming that it will be used on non planning boats such as sailboats. The 7" pitch prop would be sutable for boats that get up on plane.
Scott-"IMPULSE"87'C25/SR/WK/Din.#5688
Sailing out of Glen Cove,L.I Sound


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JB
Navigator

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USA
110 Posts
Response Posted - 08/04/2018 :  13:14:58  Show Profile
With the original 8 inch three blade propeller, and I don't know the pitch but I assume it's 7 or higher, the motor can't get anywhere near the rpms for full power output. The boat digs in around half throttle. For cruising I like to be between 5 and 5.5 knots and that's just over 1/3 throttle. Just past "Restart".

At cruising speed the current propeller isn't optimal but it hasn't really been a problem. I'm at a new marina and with the prevailing wind and water hazards, I need to be able to stop quickly. I've had a couple of close calls trying to stop. A quick stop is possible but the propeller cavitates a lot. Because the motor is on the opposite side of the dock and I have to turn around to shift and run the throttle, I'm looking for ways to make docking go smoother.

A slightly larger propeller with four bigger ears, I have read, should make it easier to stop and put the motor closer into power band at the high end. My main concern was that 5 pitch might not get me to 5.5 knots without being at max or near max throttle.

That doesn't seem to be the case based on the experience of some of the posters here. (Assuming Tohatsu hasn't re-geared the motor since 2006)

1988 C25 Wing Keel Std Rig Tohatsu 9.9 Tiller Steering and 2003 C250 Wing Keel Std Rig Inboard Diesel Wheel Steering
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Dave5041
Former Mainsheet Editor

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USA
3754 Posts
Response Posted - 08/04/2018 :  13:36:26  Show Profile
My Mercury Bigfoot 9.9 has a 10x7px4 blade that works well at low speed and cruises at 5 kt around half throttle, but the Bigfoot has about 15% more gear reduction than the standard 9.9. Either should be ok. The five will burn more fuel with higher rpm in exchange for better low speed response. Im not sure that the response difference will be noticeable.

Dave B. aboard Pearl
1982 TR/SK/Trad. #3399
Lake Erie/Florida Panhandle
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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.